In many applications of lasers or other radiation sources it is important to prevent reflected radiation from interacting with the source, since such interaction can, for instance, generate noise and unwanted feedback. An example of an application in which there frequently is a need to isolate a source from reflected radiation is lightwave communications, especially high bit rate communications over relatively long distances.
It has long been known that the Faraday effect in magneto-optic materials can be used to provide a non-reciprocal device that can serve as an isolator, i.e., a device which permits light passage in only one direction. Yttrium iron garnet (YIG) is a magneto-optic material that has been used for isolator applications. However, YIG has recently become quite expensive. Furthermore, it has only a relatively small specific rotation in the near infrared wavelength regime of interest for lightwave communications (e.g., 0.8-1.6 .mu.m), such that a large path length (about 2.7 mm for 1.31 .mu.m radiation) is required to provide the 45.degree. rotation of the plane of polarization that is necessary for an isolator. It also has a high saturation magnetization, which typically requires the use of a large, high field (e.g., SmCo) magnet that typically is not only expensive but also may affect, and be affected by, nearby components.
It is known that Bi substitution in rare earth iron garnets can greatly increase the specific Faraday rotation of these materials. Relatively thin films (of order 10 .mu.m) of Bi-substituted garnets were grown on appropriate substrates by liquid phase epitaxy (LPE) for magnetic bubble devices. However, for magneto-optic isolators relatively thick (e.g., of order 300 .mu.m) films have to be grown, and this poses difficult problems, due typically to significant differences in thermal expansion between the substrate material and the garnet film grown thereon. These differences are thought to be the cause of the frequently observed cracking of such films. Such cracking obviously decreases yield and thus results in increased cost.
Cracking is known to occur less frequently if the garnet film is grown on only one side of the substrate, since stress can be relieved through bowing of the substrate/film combination. However, such one-sided LPE growth typically poses difficult control problems, due to increased growth time and surface agitation of the melt. Such one-sided films thus are relatively difficult to manufacture. K. Nakajima et al., IEEE Transactions on Magnetics, Vol. 24(6), pp. 2565-2567, and K. Machida et al., Proceedings of the International Symposium on Magento-Optics, Journal of the Magnetics Soc. of Japan, Vol. 11, Supplement, No. Sl, pp. 347-351, both report one-sided growth of (YbTbBi).sub.3 Fe.sub.5 O.sub.12 films on a garnet substrate.
In view of the technological importance of garnet films on a substrate for use in magneto-optic isolators, it would be highly desirable to have available such films that have relatively high specific rotation, relatively small temperature dependence, and relatively low saturation magnetization that are relatively easy to manufacture, and that, significantly, are less subject to fracture than comparable prior art films. This application discloses such films, and an advantageous method of producing them.